1. Field of the Invention
The present invention relates to a stencil printing machine which performs an image transfer by conveying a print sheet while applying printing pressure to a rotating drum, and particularly, relates to a technology for peeling off the print sheet from the drum and a technology for feeding the print sheet between the drum and a pressure roller.
2. Description of the Related Art
This type of stencil printing machine includes a scanner unit which reads image data of an original to be printed, a stencil making unit which perforates a stencil sheet based on the image data read by the scanner unit, a printing unit which attaches the stencil sheet prepared by the stencil making unit onto the drum and transfers an image on the print sheet by conveying the print sheet while applying printing pressure to the rotating drum, a paper feed unit which feeds the print sheet to the printing unit, a paper discharge unit which discharges the print sheet on which the image is printed by the printing unit, and a stencil disposal unit which removes the stencil sheet attached onto the drum. Then, means which uses an instantaneous separation blow for surely peeling off the print sheet which is conveyed together with the rotation of the drum and has passed through an image transfer position is disclosed in FIG. 1 and FIG. 2 (Japanese Patent Laid-Open Publication No. 2003-136823).
The means is described by use of FIG. 1. A drum 100 has a stencil sheet 101 attached onto an outer peripheral surface 100a thereof, and rotates in a direction of an arrow A by driving force of a drum motor (not shown). A pressure roller 102 is provided at a position below the drum 100 so as to be freely movable, and presses a fed print sheet 103 to the drum 100 to apply the printing pressure thereto. A separation claw 104 is placed at a position which is downstream of the image transfer position of the drum 100 and near the outer peripheral surface 100a of the drum 100, and on a tip of the separation claw 104, a blast nozzle (not shown) is provided. The blast nozzle is connected to a separation pump 106 through a blast pipe 105, and the separation pump 106 is adapted to generate the instantaneous blow in synchronization with the rotation of the drum 100 by using the drum motor as a drive source.
In the configuration described above, the print sheet 103 is conveyed in a sheet conveying direction 107 while the printing pressure is being applied to the outer peripheral surface 100a of the rotating drum 100, and the image is transferred from the stencil sheet 101 to the print sheet 103 in this conveying process. Then, when a tip of the print sheet 103 in the conveying direction comes to a rotation position where the image transfer is completed, the separation blow is blown instantaneously from the tip of the separation claw 104 to between the tip of the print sheet 103 in the conveying direction and the outer peripheral surface 100a of the drum 100. By the separation blow, the tip of the print sheet 103 in the conveying direction is peeled off from drum 100. The tip side of the print sheet 103 in the conveying direction, which has been peeled off from the drum 100, is guided to a lower side of the separation claw 104 as shown in FIG. 2, and the print sheet 103 guided below the separation claw 104 is sequentially peeled off from the drum 100. The print sheet 103 peeled off from the drum 100 is conveyed to a predetermined discharge passage.
Specifically, because the print sheet 103 immediately after the image is printed thereon adheres to the drum 100 owing to viscosity of ink, the strong separation blow is blown to between the tip of the print sheet 103 in the conveying direction and the outer peripheral surface 100a of the drum 100 to separate the tip side of the print sheet 103 from the drum 100 forcibly. Then, the tip of the print sheet 103 in the conveying direction, which is separated from the drum 100, is guided by the separation claw 104, and thus the print sheet 103 is surely peeled off from the drum 100.
However, in the conventional stencil printing machine described above, there has been a possibility that the print sheet 103 is brought into contact with the separation claw 104, and there has been a problem that the contact of a printing surface of the print sheet 103 with the separation claw 104 causes stain of the printing surface. Moreover, the separation claw 104 is placed near the drum 100, and accordingly, there has been a possibility that the separation claw 104 pierces the drum 100 when a trouble occurs. Furthermore, the blasting from the separation pump 106 is performed only at timing of peeling off the tip side of the print sheet 103 in the conveying direction from the drum 100, and accordingly, there has been a possibility that timing when the respective spots downstream of the tip side of the print sheet 103 in the conveying direction are peeled off from the drum 100 is not stabilized, leading to generation of unevenness of the image owing to unevenness of the peeling-off timing.
Moreover, as another configuration of the paper feed unit of the conventional stencil printing machine, there is one shown in FIG. 3 (Japanese Patent Laid-Open Publication No. H2-43145 (published in 1990)).
The paper feed unit is briefly described by use of FIG. 3. A paper feed unit 200 includes a paper feed tray 202 on which a large number of print sheets 201 are loaded, a primary paper feed roller 203 placed at an upper position of the paper feed tray 202, a pair of secondary paper feed rollers 204a and 204b placed on the downstream side of the primary paper feed roller 203 in the conveying direction, and a lower guide member 205 and an upper guide member 206, which guide a lower side and upper side of each conveyed print sheet 201. The lower guide member 205 is placed from the paper feed tray 202 through the lower secondary paper feed roller 204a toward the vicinity of a pressure roller 207 and a drum 208. Moreover, the upper guide member 206 is placed only between the primary paper feed roller 203 and the upper secondary paper feed roller 204b and only in the periphery of the downstream side of the upper secondary paper feed roller 204b in the conveying direction.
Next, a paper feed operation is described. When the primary paper feed roller 203 is rotated, the uppermost print sheet 201 pressed onto the primary paper feed roller 203 is conveyed, the conveyed print sheet 201 is guided by the lower guide member 205 and the upper guide member 206, a tip of the print sheet 201 abuts on the pair of secondary paper feed rollers 204a and 204b at a contact point thereof, and the print sheet 201 is conveyed to a position where a rear end side of the print sheet 201 bends.
Next, when the pair of secondary paper feed rollers 204a and 204b are rotationally driven at predetermined timing, the bent print sheet 201 enters between the pair of secondary paper feed rollers 204a and 204b by return force thereof to return straight, and the print sheet 201 is fed between the drum 208 and the pressure roller 207 in synchronization with the rotations of the pair of secondary paper feed rollers 204a and 204b. 
In this conventional example, stable paper feed is performed by regulating the upper and lower surfaces of the print sheet 201 by the lower guide member 205 and the upper guide member 206.
However, it is necessary to set a gap between the lower guide member 205 and the upper guide member 206 at a dimension sufficiently larger than thickness of the print sheet 201 such that the gap does not resist the conveyance of the print sheet 201. Accordingly, the tip side of the print sheet 201 can freely bend within a range of the gap dimension. Moreover, though the lower guide member 205 can be extended toward the vicinity of the pressure roller 207, the upper guide member cannot be extended toward the vicinity of a position where the print sheet 201 enters between the drum 208 and the pressure roller 207 because of the presence of the drum 208 with a large diameter. Accordingly, the tip side of the print sheet 201 can freely bend to the drum 208 side. Owing to these facts, there has been a possibility that the print sheet 201 is fed between the drum 208 and the pressure roller 207 in a state where the tip side thereof is bending. When the print sheet 201 is fed in the state as described above, there has been a possibility that a wrinkle and a crease occur in the print sheet 201. Moreover, when the print sheet 201 largely bends to the drum 208 side by rotational suction force of the drum 208 and the like, there has been a possibility that the print sheet 201 contacts the drum 208 before being fed between the drum 208 and the pressure roller 207, thereby generating an image ghost on the print sheet 201.